1). Field of the Invention
The present invention relates to two- and three- component aqueous polyurethane dispersions which are cured at ambient temperatures and to the coatings prepared therefrom which have excellent chemical resistance without reduction in abrasion resistance and flexibility. More particularly, the present invention relates to a coating composition comprised of a hydroxy functional aliphatic polyurethane dispersion in water combined with an aromatic polyether and crosslinked with an aliphatic or aromatic polyisocyanate compound. More specifically the aromatic polyether can be an epoxy resin or a phenoxy resin.
2) Prior Art Description
Aqueous polyurethane dispersions and their use for the production of coatings are known. The dispersions may be cured at ambient temperature by evaporation of water and coalescence of the individual polyurethane particles. These water-based products have been developed in an effort to reduce the amount of organic solvents which are present in corresponding solvent-based coating compositions. Even though the prior art dispersions possess many valuable properties, it has not been possible to obtain coatings which possess all of the properties of coatings obtained from solvent-based coating compositions, especially with regard to hardness and solvent resistance.
The known aqueous polyurethane dispersions do not possess the amount of crosslinking which is required to obtain these properties. One method of increasing the amount of crosslinking is to blend the polyurethane dispersion with a water dispersible, blocked polyisocyanate as disclosed in U.S. Pat. No. 4,098,933. U.S. Pat. No. 4,608,413 discloses the use of water dispersible, blocked polyisocyanates in combination with polyurethanes which contain hydroxyl groups as crosslinking sites. While the systems disclosed in these patents make it possible to obtain improved hardness and crosslinking, they suffer from the disadvantage that the coating compositions must be heated to high temperatures in order to release the blocking agent and initiate crosslinking. Obviously, these coating compositions are not suitable for application to substrates which cannot withstand the unblocking temperature of the polyisocyanate.
Accordingly, it is an object of the present invention to provide aqueous polyurethane dispersions which may be cured at ambient temperature to provide coatings with excellent hardness, flexibility, solvent resistance and surface appearance.